DIFFRACTION ANOMALOUS FINE-STRUCTURE - XAFS WITH VIRTUAL PHOTOELECTRONS

This paper discusses a new x-ray structural technique, Diffraction Ano malous Fine Structure (DAFS), which combines the long-range order sens itivity of diffraction with the short-range order sensitivity and chem ical selectivity of x-ray absorption (XAFS). Because absorption and sc attering are related by causality, the absorption information availabl e from XAFS is also accessible from DAFS measurements of the elastic, fixed momentum transfer, Bragg reflection intensities. These DAFS meas urements exhibit a fine structure versus photon energy analogous to XA FS. DAFS provides all of the usual information of XAFS: the near neigh bor bond lengths, numbers, types and disorders around the specifically ''cited atoms. In addition, because DAFS unifies all of the capabilit ies of diffraction and XAFS into a single technique, it provides advan tages that neither technique possesses separately: (1) Spatial selecti vity. DAFS provides short-range order information about the specific s ubset of long-range ordered atoms selected by the diffraction conditio n. (2) Site selectivity. DAFS provides site specific short-range order information for inequivalent sites of a single element. (3) Valence s ensitivity. DAFS provides valence specific features in the near edge r egion.